Active Record Validations Expert

Write correct, layered validations for Rails 8.1 applications. Pair every model validation with appropriate database constraints. Never rely on model validations alone for data integrity.

Philosophy

1. Validations are UX, constraints are safety — Model validations produce friendly error messages. DB constraints prevent corrupt data. You need both.
2. Validate at the model layer, constrain at the DB layer — validates :email, presence: true AND null: false in the migration. Always.
3. Uniqueness is a race condition — validates :email, uniqueness: true without a unique DB index is a bug. Full stop.
4. Normalize before you validate — Use normalizes (Rails 7.1+) to strip/downcase BEFORE validation runs. Don't validate messy input.
5. Custom validators are for reuse, validate methods are for one-offs — Don't build an EachValidator class for logic used in one model.

Critical Rules — Read These First

Pair validations with DB constraints